Conventionally, a variety of primary and secondary batteries have been used in portable compact electric equipment. However, due to recent improvements in performance of such equipment, power consumption has become greater, making it difficult to provide a primary battery that can supply sufficient energy while being compact and lightweight. Although a secondary battery has the advantage that it can be repeatedly recharged and used, the amount of energy that is usable in a single charging is even less than that of a primary battery. Further, recharging a secondary battery requires a separate power source and generally takes several tens of minutes to several hours, so that it is difficult to use a secondary battery at once at any time and in any location. Although it is expected that in the future the electric equipment will become even smaller and lighter and the wireless network environment is being developed, increasing the tendency of using portable equipment, it will be difficult for conventional primary and secondary batteries to supply sufficient energy for driving equipment.
As a solution to such problems, compact fuel cell systems are drawing attention. Fuel cell systems have hitherto been developed as a driving source for large generators and automobiles. The major reason for this is that a fuel cell system has a higher power generation efficiency and produces clean wastes, as compared with conventional power generation system. A fuel cell system is also useful as a driving source for compact electric equipment, because the amount of energy that a unit volume or weight of a fuel cell can supply is several to nearly ten times that of a conventional battery. Further, because a fuel cell system can operate continuously only by refilling or exchanging fuel, recharging thereof does not take much time unlike other secondary batteries.
While various types of fuel cell systems have been developed, for compact electric equipment, especially mobile equipment, polymer electrolyte fuel cell systems, for example, are suitable. This is because these systems have the advantages of being usable at a temperature close to ordinary temperature and being safe to carry around since the electrolyte is not a liquid, but is a solid.
While fuel cell systems are based on the simple principle of generating power by supplying fuel and an oxidizer to a fuel cell, in order to achieve optimum power generation, a number of controls are carried out.
A fuel cell in a polymer electrolyte fuel cell system is structured such that a polymer electrolyte membrane, which serves as an ionic conductor, is sandwiched and held by a fuel electrode and an oxidizer electrode having catalyst layers. The polymer electrolyte membrane contains water in its interior, acting to conduct hydrogen ions (protons) and also to prevent cross-leaking between the fuel gas and the oxidizer gas. However, the ionic conductivity, which determines the performance of a polymer electrolyte fuel cell system, is greatly affected by the wettability of the polymer electrolyte membrane. In particular, the conductivity dramatically drops due to drying of the polymer electrolyte membrane, so that an increase in the internal resistance will considerably degrade the characteristics of the fuel cell system.
Therefore, power generation for a polymer electrolyte fuel cell system requires the polymer electrolyte membrane to be suitably moist for effective ionic conduction. As disclosed in Japanese Patent Application Laid-Open Nos. 2001-102059 and H08-306375, a conventional method humidifies a polymer membrane by moistening the fuel beforehand. Further, as disclosed in Japanese Patent Application Laid-Open Nos. 2001-102059 and H11-045733, another method uses water obtained as a result of power generation to moisten the fuel.
However, in fuel cell system constructions such as described above, when the fuel cell system is large, methods that are used to humidify the fuel gas with water produced during power generation require a pump for transporting the generated water, because the fuel cell is distant from the fuel supply unit. When a pump is used, power has to be supplied to the fuel cell system, so that such a system has the disadvantages of being larger and more complex.
Even for small fuel cell systems, when water generated in an oxidizer electrode is supplied to a fuel passage on a fuel electrode side, it is difficult to prevent the oxidizer from mixing with the fuel.
In addition, in polymer electrolyte fuel cell systems, hydrogen ions that have passed through a polymer electrolyte membrane serving as an ionic conductor react at an oxidizer electrode with an oxidizer (oxygen), thereby generating water at the surface of the oxidizer electrode. Because an oxidizer passage at the oxidizer electrode is narrow, unless the generated water is removed the passage is closed off by water droplets, whereby the oxidizer cannot be efficiently led to the oxidizer electrode. To prevent this, Japanese Patent Application Laid-Open Nos. 2001-102059 and 2001-160406 disclose that by using a conductive, water-repellant porous material for the oxidizer electrode, the oxidizer electrode can be prevented from becoming excessively wet. In addition, Japanese Patent Application Laid-Open No. 2001-93539 discloses that by applying a hydrophilic coating to the surface of a separator that forms a gas passage, the passage can be prevented from being closed off by generated water.
However, in these methods it was difficult to quickly remove water generated in an oxidizer electrode. These methods also suffered from the problem that when an attempt was made to lead the generated water in a certain direction, an apparatus using an electric power such as a pump or blower was required, thus enlarging the system. For this reason, it was also difficult to store the used water in a given place.